body {
    margin: 0;
    height: 100vh;
    display: flex;
    align-items: center;
    justify-content: center;
    background-color: skyblue;
}

.container {
    width: calc(40px * calc(var(--side-length)));
    height: calc(40px * calc(var(--side-length)));
    position: relative;
}

.container .horizontal,
.container .vertical {
    position: absolute;
    top: 0;
    left: 0;
    display: grid;
    grid-template-columns: repeat(var(--side-length), 1fr);
}

.container .horizontal span {
    width: 40px;
    height: 10px;
    background-color: white;
    margin: 15px 0;
}

.container .vertical span {
    width: 10px;
    height: 40px;
    background-color: white;
    margin: 0 15px;
}